When a signal is conveyed by an optical fiber, it is subjected to certain kinds of distortion, such as amplitude, frequency, or phase distortion. In order to recover a signal that is as similar as possible to the signal as emitted, it is then necessary to pass the optical signal through a regeneration device.
Devices are already known in the state of the art for regenerating an optical signal carrying information encoded by amplitude modulation of said signal, e.g. by using saturable absorbers.
Unfortunately, in present-day optical transmission devices that enable rates of 40 gigabits per second (Gbits/s) or more to be obtained, ever increasing use is being made of signals that are phase-modulated, in particular by modulation of the differential phase shift keying (DPSK) type. In that type of modulation, information is encoded in the phase of the signal: for example a “1” bit is encoded by inverting the phase of the carrier signal, while a “0” bit is encoded by a lack of phase change.
Present optical regeneration devices do not enable signals carrying information encoded by phase modulation to be regenerated correctly since such devices act only on signal amplitude. Consequently, the phase distortion of signals which degrades the information conveyed thereby cannot be eliminated or at least reduced.